How far from camera person should stand?

[agentas]

Homework Statement

A 35 mm camera has a lens with a 50 mm focal length. It is used to
photograph an object 175 cm in height, such that the image is 30 mm high.
(a) How far from the camera should the person stand?
Answer is
The person should stand 297 cm away from the camera.

Homework Equations

1/f=1/o+1/i
m=i/o

The Attempt at a Solution

1/50=1/o+1/i
m=30/175=i/o
I really do not know how to relate hight of the person to distance to the camera .please help someone!How to get this answer?297cm?

 

[tiny-tim]

Hi agentas!

A 35 mm camera has a lens with a 50 mm focal length. It is used to
photograph an object 175 cm in height, such that the image is 30 mm high.
…
m=30/175=i/o

nooo … 3/175 … look at the units!

Try again! ​

 

[agentas]

Hi agentas!


nooo … 3/175 … look at the units!

Try again! ​

you look yourself. Iam using cm instead of meters.What does not change anything.

 

[tiny-tim]

you look yourself. Iam using cm instead of meters.

Yes, 3cm/175cm.

 

[rl.bhat]

If u is the object distance and v is the image distance, then
m = v/u = i/o.
Now 1/f = 1/u + 1/ v
Multiply by u to both side. you get
u/f = 1 + u/v = 1 + o/i.
Now solve for u.

 

[DaveC426913]

Uh guys? Zero.

The person should stand close enough to the camera to look through the viewfinder!

Of course, that doesn't say anything about how close the object should be...

 

[agentas]

If u is the object distance and v is the image distance, then
m = v/u = i/o.
Now 1/f = 1/u + 1/ v
Multiply by u to both side. you get
u/f = 1 + u/v = 1 + o/i.
Now solve for u.

thanks I got the right answer :)